1 Answer
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If you only want to backup your data, you should boot an Ubuntu Live CD maybe in an older version ( http://mirror.anl.gov/pub/ubuntu-iso/DVDs/ubuntu/ ) and copy the data from the disk to an external drive (USB-HDD, Flash-Disk,...)
Most important is in /home/username

about 1: sorry my mistake, the original OS is ubuntu 11.04 about 2: I'm trying to recovery by Live CD, but after mounted /dev/sda1 to /mnt and chroot / /mnt, I found /home direcory is empty..... about 3: The keyboard is freezed, cannot change to the console about 4: Yes it is intel on-board
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Freeman ZhangMay 2 '12 at 7:03

OK, my /home is in /dev/sda2. it is safe to reinstall ubuntu 11.04?
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Freeman ZhangMay 2 '12 at 7:14

their should be no problem to install 12.04 if the live cd boots properly. You can mount /dev/sda2 to /home during installation. But my advice: rename the username folder in /home to username.old or so and install the system in sda1. Then during the installation mount sda2 in /home and create your user. After installation is finished, copy the needed files from username.old to username. That's because after Upgrade(s) there is also much waste in the home directory and so you can clean it. It's more work, but the cleaner way I think.
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Felix HallerMay 2 '12 at 16:23